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SCHEDULE 13D/A 0001802528 XXXXXXXX LIVE 8 Common Stock, $0.001 par value 11/13/2025 false 0001622229 19240Q201 Cogent Biosciences, Inc. 275 Wyman Street 3rd Floor Waltham MA 02451 Ms. Erin O'Connor (267) 262-5300 Fairmount Funds Management LLC 200 Barr Harbor Drive, Suite 400 West Conshohocken PA 19428 0001802528 N Fairmount Funds Management LLC a AF N DE 0.00 15877418.00 0.00 15877418.00 15877418.00 N 9.9 IA The securities include (a) 9,003,418 shares of common stock, $0.001 par value ("Common Stock"), and (b) 6,874,000 shares of Common Stock issuable upon conversion of approximately 27,496 shares of Series A Convertible Preferred Stock, par value $0.001 per share (the "Series A Preferred Stock") directly held by Fairmount Healthcare Fund II L.P. ("Fund II"). The securities exclude shares of Common Stock issuable upon conversion of shares of Series A Preferred Stock held directly by Fund II in excess of the beneficial ownership limitation of 9.9%. Row 13 is based on 160,379,562 shares of Common Stock outstanding as of November 13, 2025, consisting of (i) 143,376,529 shares of Common Stock outstanding as of November 5, 2025, as reported in the Company's most recent Quarterly Report on Form 10-Q, (ii) 11,129,033 shares of Common Stock sold in the offering, as reported in the Company's Current Report on Form 8-K filed on November 13,2025, and (iii) 6,874,000 shares of Common Stock underlying the 27,496 shares of Series A Preferred Stock owned by Fund II, subject to the beneficial ownership limitation. 0001830382 N Fairmount Healthcare Fund II GP LLC a AF N DE 0.00 15877418.00 0.00 15877418.00 15877418.00 N 9.9 OO The securities include (a) 9,003,418 shares of common stock, $0.001 par value ("Common Stock"), and (b) 6,874,000 shares of Common Stock issuable upon conversion of approximately 27,496 shares of Series A Convertible Preferred Stock, par value $0.001 per share (the "Series A Preferred Stock") directly held by Fairmount Healthcare Fund II L.P. ("Fund II"). The securities exclude shares of Common Stock issuable upon conversion of shares of Series A Preferred Stock held directly by Fund II in excess of the beneficial ownership limitation of 9.9%. Row 13 is based on 160,379,562 shares of Common Stock outstanding as of November 13, 2025, consisting of (i) 143,376,529 shares of Common Stock outstanding as of November 5, 2025, as reported in the Company's most recent Quarterly Report on Form 10-Q, (ii) 11,129,033 shares of Common Stock sold in the offering, as reported in the Company's Current Report on Form 8-K filed on November 13,2025, and (iii) 6,874,000 shares of Common Stock underlying the 27,496 shares of Series A Preferred Stock owned by Fund II, subject to the beneficial ownership limitation. Common Stock, $0.001 par value Cogent Biosciences, Inc. 275 Wyman Street 3rd Floor Waltham MA 02451 This Amendment No. 8 amends and supplements the statement on Schedule 13D originally filed with the Securities and Exchange Commission on July 6, 2020, Amendment No. 1 filed on June 21, 2022, Amendment No. 2 filed on June 13, 2023, Amendment No. 3 filed on February 16, 2024, Amendment No. 4 filed on March 27, 2024, Amendment No. 5 filed on April 1, 2024, Amendment No. 6 filed on June 13, 2024, and Amendment No. 7 filed on July 14, 2025 (as amended, the "Statement" or the "Schedule 13D") by the Reporting Persons with respect to the Common Stock of Cogent Biosciences, Inc. (the "Company"). Unless otherwise defined herein, capitalized terms used in this Amendment No. 8 shall have the meanings ascribed to them in the Statement. Unless amended or supplemented below, the information in the Statement remains unchanged. The information in rows 11 and 13 of each of the cover pages (including the explanatory note in the "Comments") of this Amendment No. 8 is incorporated by reference herein. The information in rows 7 through 10 of each of the cover pages of this Amendment No. 8 is incorporated by reference herein. Lock-Up Agreement In connection with the Company's underwritten public offering of common stock that closed on November 13, 2025 and the Company's underwritten public offering of convertible senior notes that is expected to close on November 18, 2025, Fairmount Funds Management entered into a customary lock-up letter agreement (the "Lock-Up Agreement") with J.P. Morgan Securities LLC, Jefferies LLC, Leerink Partners LLC and Guggenheim Securities, LLC as representatives (the "Representatives") of the several underwriters to an underwriting agreement with the Company. Pursuant to the Lock-Up Agreement, Fairmount Funds Management agreed to refrain from selling shares of the Company's securities without the consent of the Representatives, and other customary lock-up conditions, for a period of 45 days after the date of the Company's final prospectus supplement related to the offerings. The Reporting Persons did not purchase any securities of the Company or otherwise participate in the offerings. The foregoing description of the Lock-Up Agreement does not purport to be complete and is qualified in its entirety by reference to the form of the Lock-Up Agreement, which is filed as Exhibit 99.2 to this Schedule 13D and incorporated herein by reference. 99.1 Joint Filing Agreement 99.2 Lock-Up Agreement (incorporated by reference to Exhibit A to the Underwriting Agreement filed as Exhibit 1.1 of the Company's Form 8-K filed with the SEC on November 13, 2025). Fairmount Funds Management LLC /s/ Peter Harwin Peter Harwin, Managing Member 11/17/2025 /s/ Tomas Kiselak Tomas Kiselak, Managing Member 11/17/2025 Fairmount Healthcare Fund II GP LLC /s/ Peter Harwin Peter Harwin, Managing Member 11/17/2025 /s/ Tomas Kiselak Tomas Kiselak, Managing Member 11/17/2025